In principle, magnetic sensing technologies can be used by divers or autonomous underwater sensing platforms (e.g., Autonomous Unmanned Vehicles (AUVs)) for detection, localization and classification (DLC) of magnetic objects. For example, underwater mines in littoral “very shallow water/surf zone” (VSW/SZ) environments constitute magnetic objects that may be located in the sea bottom, tethered at mid depth or floating at the surface. Therefore, fully autonomous magnetic DLC of all underwater mines requires magnetic sensing systems that will function effectively onboard highly-mobile sensing platforms such as free-swimming divers or small robotic submarines that are capable of unconstrained three-dimensional motion. Accurate identification of mine-like objects often requires the sensing platform to approach very close to the object. Therefore, it is desirable that magnetic sensing system be able to guide the sensing platform toward the object, i.e., “to home in on” the object. Other potential applications for magnetic sensing systems that involve similar unconstrained three-dimensional sensing platform motion include small robotic flying craft using magnetic sensors to remotely detect, localize and home in on magnetic objects such as land-based mines, camouflaged enemy tanks or even hidden nuclear facilities. In practice, however, the mobile magnetic sensing art has been limited by the fact that the very small magnetic signals of magnetic objects are convolved within the much larger background magnetic field of the Earth.
Magnetically polarized objects or targets such as underwater mines create characteristic dc magnetic field anomalies within the relatively constant background magnetic field of Earth. Magnetic sensing systems can detect the magnetic anomalies or target “signatures” and use the magnetic signature data to detect, localize and classify the mines. It is well-known that at distances greater than two or three times an object's linear dimensions, its magnetic signature (measured in “Tesla”) approximates that of a dipole with well-defined mathematical characteristics. However, magnetic dipole field magnitudes decrease with the inverse cube of distance. Thus, at object-to-sensor distances of a few meters, a mine-like object's magnetic signature strength rapidly becomes very small (i.e., on the order of nano-Tesla or 10−9 T, or even pico-Tesla or 10−12 T) in comparison to the Earth's 50 micro-Tesla (or 10−6 T) background field. As a result, a basic challenge for mobile magnetic sensors is the need to discriminate the very small dc target signature components that are convolved with the relatively very large field of Earth. Sensor motion in the very large Earth field can cause relatively huge, orientation-dependent changes in measured vector field components. The large, non-target-related changes in measured field components can overwhelm the relatively small target signatures and thereby reduce the sensor's effective DLC range.
For small autonomous sensing platforms, additional challenges to effective use of magnetic sensors (for DLC of magnetic objects/targets) derive from the platforms' small size, power and computational budgets. The platform constraints require that the magnetic sensors be small and operate well with small power and computational budgets. Further, in littoral VSW/SZ environments, target localization range can be reduced as the of ten turbulent, three-dimensional nature of these environments typically will cause large changes in sensor system orientation that will exceed the motion tolerance capability of conventional magnetic sensor approaches. Still further, the operational constraints that are imposed by the naval diving environment largely preclude the practical use of conventional prior art magnetic sensor system designs and methods based on magnetic scalar total field or magnetic vector/gradient tensor technologies.
In order to meet the challenges of providing practical and effective magnetic target DLC capabilities for small, highly-mobile maneuverable sensing platforms, U.S. Pat. No. 6,476,610, (i.e., “the '610 patent” as it will be referred to hereinafter) teaches a novel magnetic anomaly gradient sensing system and signal processing concept. The disclosed approach is based on the use of vector triaxial magnetometers (TM) for magnetic field sensing, and the use of triaxial accelerometers for measurement of sensing platform motion and orientation.
Briefly, the '610 patent discloses a target localization approach denoted as Scalar Triangulation and Ranging (STAR). The STAR method uses simplified scalar “contractions” of magnetic gradient tensor components to determine relative distances to an object, i.e., “triangulate” the object's location. The symmetry properties of the gradient contraction, combined with the '610 patent's sensor array geometries, help to mitigate the adverse effects of large changes in sensor platform orientation. Also, the mathematical simplicity of the STAR approach requires relatively little computer processing power and is, therefore, relatively easy to implement onboard small autonomous highly maneuverable sensing platforms. Furthermore, as disclosed in a subsequent related U.S. patent application Ser. No. 10/373,493, filed Feb. 19, 2003, (i.e., “the '493 application” as it will be referred to hereinafter), the symmetry properties of the gradient contraction scalars measured by each single “axis” (i.e., each set of two TMs) can be exploited to provide robotic Underwater Bottom Vehicles (UBVs) with robust two-dimensional magnetic anomaly guidance for homing in on magnetic targets.
While gradient contraction-based DLC of magnetic targets is a promising approach for highly-mobile sensing platforms, the approaches of the '610 patent and the '493 application have the following shortcomings with regard to efficient DLC of magnetic targets:
(1) The '610 patent does not provide robust target localization information for all three dimensional sensor-target orientations. That is, for certain orientations of the sensor embodiments of the '610 patent with respect to the target, the sensor-target distance (i.e., range) may be ambiguous or indeterminate.
(2) The particular implementations of the '610 patent's and the '493 application's STAR method are based on comparisons of partial gradient contractions of single-axis array elements that do not provide an accurate measurement of the target's vector dipole moment strength, M. However, the true magnitude and direction of M constitute the basis of a target's magnetic signature thereby providing an important basis for target classification. Accurate measurements of M are required to distinguish between real mines and underwater magnetic debris such as, for example, “tin cans.” Thus, the lack of robust dipole moment classification can lead to frequent “false alarms” with regard to the detection of non-mine-like targets and consequently require inefficient use of maneuverable platform resources if they are used to home in on undesired targets. Note that inaccurate measurements of M can also result in inaccuracies in target localization.
(3) The sensor system embodiments and methods disclosed in the '493 application are essentially limited to two dimensional magnetic guidance (where the sensing platform and target are nearly in the same plane) and do not provide explicit target location and classification information. On the other hand, while the '610 patent is not limited to two dimensional platform motion, it does not develop magnetic guidance signals that are easily used by a vehicle operator or controller for homing in on magnetic targets.
(4) The '610 patent and the '493 application are not capable of detecting or discriminating whether the sensor is in the far field zone or the near field zone of a magnetic target. However, validity of DLC calculations depends on which target zone the sensor is located. Therefore, the far-field/near-field information is important for a sensor system that can be used to home in on a magnetic target.